The four major broadcast networks are seeing such a significant drop in prime-time viewing by 18-to-34-year-olds that some ad agency executives are saying it is approaching crisis proportions.

In a year-to-year comparison of regularly scheduled prime-time programming for the first three weeks of the new season, the four-ABC, CBS, NBC and Fox-are down 12% in the coveted demographic, said David Marans, senior partner-director of U.S. media research and resources at J. Walter Thompson USA, New York.

AN OMINOUS TREND

"It's a very ominous trend," Mr. Marans said.

"As the ratings continue to erode, and prices remain high, an advertiser has to begin to worry that it's costing him more than it's worth to reach his target audience," said another well-known media research consultant who asked to remain anonymous.

"Putting aside NBC's Thursday night lineup," Mr. Marans said, "at this point last season four regular network series had at least a 9 rating among adults 18 to 49. This year, there are none."

That key demographic group is the one a majority of TV advertisers pay a premium to reach.

The ratings gap between NBC's Thursday night lineup and the rest of the Big 4's overall lineup "is now a chasm-a canyon," he added.

`ER' THE RATINGS LEADER

In adults 18 to 49, NBC's "ER" leads all regular programs for the first three weeks with an average 16.4 rating as measured by Nielsen Media Research. The net's Thursday night lineup occupies the first five places in the demographic, with No. 5 "Veronica's Closet" garnering a 12.3 rating.

The next-highest-rated show in the demo is ABC's "The Drew Carey Show," at an 8.5 average rating thus far this season, dropping off almost 4 full ratings points from "Veronica's Closet."

Then come "NFL Monday Night Football," on ABC with an 8.3 rating; CBS' NFL football telecasts, which run over from their scheduled time into prime time on Sunday nights, at a 7.5 rating; and a tie among "Home Improvement," "Spin City" and "Just Shoot Me"-all at 7.3.

FORTUNES OF BASEBALL

If the post-season baseball games are factored in, the results aren't much better. For Fox, baseball does better than its regular season programs, Mr. Marans said, while it is dragging NBC down.

The scale of ratings erosion "is of great concern to us," said Susan Nathan, senior VP-director of media research at McCann-Erickson Worldwide, New York.

"But we must keep in mind that we're only three weeks into the season," she added. "We need to give this a chance to shake out."

Mr. Marans' concern is that younger audiences are tomorrow's future.

`WHAT ARE YOU LEFT WITH?'

"If you're bleeding in 18-to-34-year-olds, you are in serious trouble in the not-too-distant future," the JWT executive said. "The advertisers who provide the sole revenue source for the networks have largely ignored the 55-plus audience, which CBS owns. So if the 18-to-34-year-olds are walking away from the Big 4 networks, what are you left with? The four of them competing for the small niche of 35-to-54-year-olds?"

David Poltrack, exec VP-research and planning for CBS, said the networks are facing a programming dilemma.

"Advertisers pay a premium for 18-to-34-year-olds, but that demographic category watches less TV and is a declining percentage of the population," he said.

Going forward, do the networks create most of their programming to land that demo or do they pursue 35-to-54-year-olds, who are growing in number and have more discretionary income, with the hope that Madison Avenue will eventually see the light?

It's expected that broadcast network TV viewership will erode another 4% this year, bringing its share of audience to 57%. And ad buyers are wondering at what point the continuing erosion will stop.

"I used to think at 50%," said one network executive. "But now I'm of the opinion that the networks might find it more profitable to go below that number, maybe even as low as 40%."

With a combination of repeats and low-cost programming, this executive said, broadcasters could make money at 40%.

CABLE'S NUMBERS RISE

"It's likely their shares would still be significantly higher than most cable programming, thus still commanding a good [cost-per-thousand] advantage," he noted.

As broadcast TV has eroded, cable TV's numbers have risen. Basic cable's household ratings overall for the season to date has risen 16%.

Other trends in the young season: CBS is overtaking Fox in adults 18 to 49, which is a psychological blow to Fox; a revitalized Friday night lineup on ABC, coupled with surprisingly poor numbers on Sundays and Tuesdays; and the almost complete collapse of fledgling network UPN.